This invention relates to a flat sealing gasket made of soft material, particularly a cylinder head gasket for internal combustion engines and is of the type which has an impregnated and, on occasion, metal-reinforced nonwoven mat provided unilaterally or bilaterally on the entire mat face or a part thereof, with a polymer coating containing, according to requirements, dissolved, suspended or dispersed liquid or solid additives.
Cylinder head gaskets made of a soft material preferably comprise nonwoven fabric mats which may be metal-reinforced. Frequently, such gaskets are, mostly for improving their strength, sealing properties and service life, impregnated with synthetic polymerizable liquids in such a manner--as disclosed, for example, in U.S. Pat. No. 3,970,322 issued July 20, 1976--that the impregnating material fills the pores of the soft material generally to approximately 60% to 90% and further, the impregnating material is in most cases thermally polymerized in the finished gasket mat.
By virtue of the polymerized impregnating agent the cylinder head gasket becomes relatively rigid and thus resists deformations to adapt it to the sealing faces of the engine block in a satisfactory manner upon installation. For eliminating this disadvantage, it has been known to provide the gasket, on one side or on both sides, on its entire surface or on one part thereof, with a preferably elastic polymer coating to improve the microseal and macroseal properties. Further, impregnated cylinder head gaskets without such coating may, during engine service, tend to adhere to the sealing face of the engine block or the cylinder head or the polymerized impregnating material in the gasket may, after a longer storage and under the effect of light, air or ozone, change and thus become brittle or may even soften. Thus, the coating provided on the gasket face or faces contains, dissolved, suspended and/or dispersed therein, substances which oppose the adhesion tendency as well as substances which protect the gasket from aging, light and/or ozone. Similarly, the coatings on the cylinder head gaskets may contain substances which, dependent upon requirements, increase or decrease sliding properties and are, in most instances, fine grained or powdery substances.
In order to sufficiently dissolve and/or bind the generally solid, powdery or fine grained additives in the coating material, it has been necessary that the polymer material of the coating be present in large quantities relative to the additives. In order to achieve that the coatings result in all the improvements which are desired of the additives, the gasket faces had to be provided with relatively thick coatings. Thick coatings, however, require a greater quantity of the usually expensive polymer material, they dry only with difficulty and are furthermore difficult to polymerize. Also, they require additional process steps and applicator devices which render the manufacture of the gaskets more expensive. The relatively thick coatings are destroyed more easily during the operation of the engine and further, they occasionally adversely affect the behavior of the seal.
In addition, good adherence of the coatings on the gasket faces is required. Impregnated gaskets, however, with a high degree of pore fill of the polymerized impregnating agent have a relatively smooth and hard surface so that an anchoring of the coatings on the soft material involves difficulties. A roughening of the soft material to enhance the adherence of the coating cannot be considered and a use of adhesives would, in turn, make the manufacture of the gasket significantly more expensive.